The invention relates to the measurement of delays of sequences of images such as film and video type signals in order to provide a measure of quality of image processing circuitry or to provide compensation processing of associated signals. In particular, it is known to subject video like signals to various types of processing which delay the signal by delays which can consist of multiple frame periods in length. It is desirable to measure this delay in order to modify the processing or transmission of the signal to keep the delay to a minimum, to delay corresponding signals by a corresponding amount to preserve mutual synchronization, or for other purposes as will become known to one skilled in the art from the teachings herein.
1. Field of the Invention
The field of the invention includes processing of sequences of images such as television video signals, motion picture film and the like by various electronic, optical and mechanical devices, which processing adds a delay to the signal. Since these image sequences are often utilized in conjunction with other sequences of events or signals, the delays which are imparted cause synchronization problems with the need to correct other signals accordingly, or otherwise to minimize the delay. Of particular interest are television programs consisting of a video signal and one or more associated audio signals where the delay of the video signal without delaying the corresponding audio signal creates a corresponding lip sync error which can be most annoying to the television program viewer. The image sequences may be conveyed in raster scan, bit mapped, compressed or any other of the various forms or formats which are well known in the art on electronic, magnetic, optical or other of the various well known storage, distribution and transmission methods.
2. Description of the Prior Art
In the motion picture film industry it is known to measure the speed of film frames or sprocket holes past a given point to synchronize the film to sound recordings, however it is believed that there are no devices which measure the relative delay of a given film frame with respect to any plurality of its positions in time.
Several video processing devices utilize internal circuitry to determine the delay of the video signal through the device. While this method is capable of considerable accuracy and low cost, it is not readily suitable for measuring the delay through multiple devices since the individual delays would need to be summed by some means. In addition, the internal circuitry to provide such measurement must be incorporated in the design upon manufacture, otherwise costly retrofitting is required. There are already numerous devices in use which do not include such internal circuitry, thus there is a need for a device which can measure delays without internal modification of existing products.
U.S. Pat. No. 4,313,135 describes an audio to video synchronization apparatus and method with a method of detecting the delay of a video signal by comparing the relatively delayed and undelayed signals in a phase locked loop type circuit coupled with a video correlator (16 of FIG. 2). The correlator performs a correlation of the delayed and undelayed video to determine if the same frame of a current (undelayed) and previous (delayed) signal are being applied to the inputs.
The '135 correlator operates such that each signal, delayed and undelayed, is sampled and the sample value stored as in a sample and hold circuit, at the center of several active video lines. The line samples are subtracted, i.e. the sample from line number X from input (undelayed) video is subtracted from the sample from X of output (delayed) video, the absolute value of the difference being a measure of the correlation or matching of the lines. The absolute value of the differences for a number of lines is averaged thus giving a voltage which is proportional to the correlation between the delayed and undelayed video signals. If the undelayed and delayed video signal present the same image frame during the frame period they will match, thus the difference voltage will be low indicating the correlation is high. If the undelayed and delayed video signal present a different image frame during the frame period they will not match due at least to random noise, then the difference voltage will be higher indicating the correlation is low. This signal is used to determine if the delay is less than a frame or greater than a frame. In the presence of NTSC color subcarrier it is known that the picture period comprises two frames (or four fields), the subcarrier being reversed on the second frame with respect to the first. This allows the correlator to distinguish delays of up to one frame, from delays between one and two frames but can not distinguish delays over two frames from delays less than two frames, since the subcarrier for frame 1 is the same phase as in frame 3.
The circuit described in '135 has difficulty distinguishing the number of frames of delay of the delayed signal once the delay exceeds one unique picture period, since the correlation when anything but a less than one unique picture period signal delay is present is always low. Consequently while the correlation system shown in '135 may distinguish between a partial frame and a one plus partial frame delay, it is unsuitable for example to use to distinguish a delay of 3 frames from a delay of one frame.
U.S. Patent No. Re. 33,535 reissued Feb. 12, 1991 with an initial issue date of Oct. 27, 1987 shows how to measure relative audio to video delays by use of timing signals encoded on the video, but does not have measurement of relative delay by use of the video itself.